Abstract:

In this paper, a pressure-swing distillation (PSD) process with two recycle streams for the separation of ternary mixture C₂H₅OH/C₄H₈O₂-3/C₄H₈O-3 was proposed. According to the residual curve map (RCM) of the ternary mixture, each of the three pairs of components form a minimum-boiling azeotrope, and the composition of C₂H₅OH/C₄H₈O₂-3 azeotrope changes significantly with pressure. Moreover, the coexistence of the three azeotropes results in a distillation boundary pinch, across which a practical distillation in normal pressure is impossible, and thus a traditional procedure to develop a PSD may be invalid. In this paper, such a difficulty was overcome by pressure optimization of the columns and a novel separation configuration was developed. Rigorous steady-state simulations of this process were carried out. Under six different pressures of the high-pressure column (column T1), a sequential iterative approach was adopted to optimize the stage numbers, feed stages, reflux ratios and other design variables in order to reduce energy and equipment costs. By comparing the economics of these six processes, the optimal pressure of the T1 column was obtained, and heat integration was applied resulting in 14.9% extra reduction in the total annual cost (TAC) for the optimal process.

Key words: ternary mixture, binary azeotrope, pressure-swing distillation, RCM, optimization, heat integration

摘要：

提出了一种带两股循环的三塔变压精馏结构用于分离乙醇（C₂H₅OH）/乙酸乙酯（C₄H₈O₂-3）/2-丁酮（C₄H₈O-3）三元混合物。由该三元混合物的剩余曲线图（RCM）可知，在大气压下该混合物的每一对组元均形成二元最低共沸物，且乙醇/乙酸乙酯二元共沸物组成随压力变化敏感。三个最低共沸物的同时存在形成了精馏边界线夹紧点现象，而通常压力下的精馏塔无法跨越，以至于传统变压精馏无法应用。通过对精馏塔压力的最优化克服了这一困难，提出了新的分离流程，并对过程进行严格稳态模拟。针对初分塔（T1）塔压不同的6种流程，采用序贯迭代法对各塔的塔板数、进料板位置、回流比等参数进行了优化。通过对比6种流程的经济评价和比较，得到了T1塔的最优压力，并经过能量集成使得过程的年度总费用降低了14.88%。

关键词: 三元混合物, 二元共沸物, 变压精馏, 剩余曲线图, 优化, 能量集成